Among the glasses used for the aesthetics or decoration of the table, crystal occupies an exceptional place: its brightness, its density, and its sonority make it a much esteemed material. In addition, it exhibits the characteristics of formability at moderate temperatures, of having a long working range, and of being able to be easily cut and polished.
It is known that this group of above numerated properties is due to the presence of lead oxide in the glass composition. But lead is a toxic element and its usage imposes so many constraints on the level of production of the products as well as its use in products designed for use in contact with food. During manufacturing, regulations govern, for example, the handling of lead-containing products and the treatment of the production wastes. In use, it is known that lead present in the glass can migrate into the food that is in contact with the glass which can lead to risks for the consumer. This migration is especially observed when the glass is in contact with an acid medium, for example, fruit juices or alcoholic beverages. Therefore, regulations limit the maximum acceptable lead release for products designed to be in contact with food.
As a rule, when the glass composition and the manufacturing process are well controlled, the amounts of lead released are very minute. However, over the past few years a tendency to strengthening the regulations has been observed. Consequently, there exists a need for glasses containing no lead, but having the properties and a manufacturing cost similar to those of crystal.
The present invention seeks to satisfy this need in disclosing glasses suitable for use as tableware and/or decorative crystal glassware meeting the following requirements:
(1) a glass density of at least 2.9 g/cm.sup.3 ; PA1 (2) an index of refraction of at least 1.545; PA1 (3) the glass displays a pleasant sonority; PA1 (4) the glass exhibits a moderate linear coefficient of thermal expansion (lower than 100.times.10.sup.-7 /.degree.C. between 0.degree.-300.degree. C.) in order not to be too sensitive to thermal shock; PA1 (5) the viscosity of the glass at high temperature is close to that of crystal; the temperature at which the glass exhibits a viscosity of 10 Pa.s (100 poises) does not exceed 1470.degree. C.; and PA1 (6) the viscosity of the glass at the liquidus temperature is greater than 600 Pa.s (6000 poises).